1. Field of Technology
The present invention generally relates to a solar cell module and, more particularly, to a package of CdS/CdTe solar cells.
2. Description of the Prior Art
As a source of electric energy, much attention is now centered on solar cells which are inexhaustible and are clean. This is because solar energy is limitless and readily available everywhere on the earth and can be directly converted into electric energy at any convenience. However, solar cells have not yet come into wide use because the cost of electricity available from solar cells is very high. The high cost of electricity is based on the high cost of manufacture of the solar cell as well as that of a module of solar cells. While many efforts have been made to reduce the manufacturing cost of solar cells, it is equally important for efforts to be made to reduce the manufacturing cost of solar cell modules. In addition, with modification in the design of the solar cells, the design of the package must also be modified, or developed, enough to accommodate the modified design of the solar cells.
Most of the prior art solar cell modules have been developed with the aim of being used with crystalline-type solar cells such as single crystal silicon solar cells and polycrystal silicon cells and, therefore, need to be modified in order for them to be useable with thin-film solar cells such as II-VI group compound semiconductor solar cells and amorphous silicon solar cells.
The II-VI group compound semiconductor solar cells include CdS/CdTe solar cells, the basic structure of which is disclosed on pages 801 to 804 of "PROCEEDINGS OF THE SIXTEENTH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE", September, 1982. One package of CdS/CdTe solar cells is disclosed in Japanese Laid-open Patent Publication No. 60-32352 (corresponding to U.S. Pat. No. 4,578,526, patented Mar. 25, 1986). Since it was found that CdS/CdTe solar cell have the following peculiar property, it requires a special package structure.
As a CdS/CdTe solar cell module, some of the inventors of the present invention have devised the module having the construction shown in FIG. 3 of the accompanying drawings. The solar cell module shown in FIG. 3 comprises a solar cell element 1 formed on a substrate 6 so as to leave a peripheral margin 20 exteriorly around the solar cell element 1, a protective layer 7 covering the solar cell element 1, a layer of synthetic resin 10 filled in the space between the solar cell element 1 and the protective layer 7, and an electrically insulating film 40 positioned between the solar cell element 1 and the resin layer 10 and held in contact with the solar cell element 1 without being bonded thereto.
Another type of CdS/CdTe solar cell module recently devised is shown in FIG. 4. The solar cell module shown in FIG. 4 is similar to that shown in FIG. 3, but differs therefrom in that no electrically insulating film 40 is employed and a gaseous medium 30, either air or a gas containing oxygen, is filled in a space delimited by the substrate 6, a metal foil 7 and the resin layer 10 bonding the substrate 6 and the metal foil 7 together.
In any of these prior art CdS/CdTe solar cell modules, much care has been taken to avoid any possible entry of a liquid medium, and no consideration has been paid to the influence which oxygen may bring about on the element when fabricated into a module. It has recently been found that oxygen present around the Cd/CdTe solar cell element greatly affects the element at high temperature. More specifically, as shown by Comparison 2 in the Table shown below, it has been found that the reduction in the conversion efficiency during operation at a high temperature without oxygen around the element is very drastic as compared with that with oxygen. The reason for it appears to be that the p-type characteristics of CdTe of the element is reduced in the absence of oxygen.
The solar cell module shown in FIG. 4 is an improved version of the solar cell module of FIG. 3 wherein attempts have been made to obviate the above discussed problem. Since in the solar cell module of FIG. 4, the gaseous medium containing oxygen is in contact with the CdS/CdTe solar cell element, the oxygen participates in lessening the speed of reduction of the conversion efficiency, but at high temperature the oxygen tends to become diminished and, therefore, the reduction in conversion efficiency is still marked.